PROJECT SUMMARY: The overarching goal of the proposed research is to investigate the neural and genetic basis of a naturally-occurring difference in aggressive behavior. Aggression has a strong genetic component, and yet we understand very little about how variation in the genome alters brain function to produce changes in behavior. Most of what is known about the relationship between genetic variants and social behavior comes from a small number of traditional lab model organisms. Using a novel natural vertebrate system, we employ an interdisciplinary approach to investigate the role of genetic polymorphisms in producing neural and behavioral variation. Rock- and sand-dwelling cichlid fishes from Lake Malawi in East Africa are closely-related and their genomes are very similar, yet they exhibit substantial diversity in social behaviors, including aggression. In Aim 1, I will quantify differences in aggressive behavior between rock- and sand- dwelling species and their hybrids. In Aim 2, I will first map the brain regions recruited during aggression by visualizing the expression of a marker of neural activity. Secondly, I will characterize the gene expression patterns of aggression-activated neurons by sequencing their transcriptomes, allowing us to discover novel genetic variants associated with differences in behavior. By comparing these results across species and their hybrids, we will provide insight into the neural and genomic mechanisms underlying phenotypic variation in aggression. In Aim 3, we will causally test the effect on brain and behavior of a candidate genetic variant in a vasopressin hormone receptor gene via CRISPR-Cas9 genome editing technology. This integrative approach is designed to provide insight into how variation in aggression can be produced by small changes to the sequence or regulation of genes. Taken together, this strategy will identify how the neural circuits regulating an essential social behavior are controlled at the level of the genome in outbred vertebrates, providing a model of behavioral diversity in humans.